In the field of gas turbines for aircraft engines, there has long been awareness of the need to increase performance by reducing weight as much as possible. In time, this has lead to the construction of arrays of aerofoils that, on the one hand are subjected to high aerodynamic loads and, on the other, have increasingly smaller thicknesses and therefore inevitably have low rigidity, both flexural and torsional.
The reduced rigidity of the aerofoils has, inevitably, resulted in the construction of turbines that have been found to be unstable under certain functional conditions. In particular, this instability is due to marked sensitivity to aeroelastic phenomena deriving from aerodynamic interactions between the aerofoils of a same turbine stage, with the consequent triggering of vibrations that stress the arrays, leading them to structurally critical conditions, as well as generating noise emissions.
This phenomenon of self-induced aeroelastic vibrations, known as flutter, thus defines a constraint in the design of arrays. Typically, aerofoils can be made more rigid to minimize this phenomenon, with a consequent increase in their weight that, as explained above, is undesirable.
As an advantageous alternative, it is known to vary, in the design of the array, the characteristics of a part of the aerofoils so as to diverge from a standard configuration of axial symmetry.
In other words, the geometry and/or the relative position of the aerofoils in each array is/are determined so as to intentionally “detune” or “mistune” the eigenfrequencies of the critical vibrations modes between a first set of aerofoils with respect to those of a second set, and to alternate the aerofoils of the first set with those of the second set to form the array.
In this way, it is found that the aerodynamic interactions between adjacent aerofoils of different types are reduced, thereby rendering the entire array more vibrationally stable.
In known solutions with aerofoils having intentionally detuned eigenfrequencies, aerodynamic efficiency usually drops. In fact, by varying the geometry on the high and low pressure sides and/or on the leading and trailing angles between aerofoils of the first and second sets, the outflow conditions (pressure, gas flow directions, etc.) in the various inter-blade channels change radically with respect to that designed in a standard type of axial-symmetric situation.
U.S. Pat. No. 4,097,192 describes a turbine rotor that is intended to reduce flutter without impairing aerodynamic efficiency. In this case, the detuning is accomplished without altering the external geometry and the pitch between the aerofoils, but by making a recess in a radial end of the aerofoils of the first set and by making the aerofoils of the second set with fully solid blades.
In this rotor, the above-stated radial ends must be free and so they are not connected to each other by any outer annular platform. However, in some applications it is opportune, or even necessary, that the rotor has an outer annular platform interconnected with the aerofoils, for which the solution of U.S. Pat. No. 4,097,192 cannot be effectively adopted.
Furthermore, the machining for removing material and making the recesses at the radial end of a part of the aerofoils takes extra production time and costs.
Another drawback of known solutions lies in the fact that the aerofoils of the first and second sets, by being individually produced with different geometrical characteristics, require dedicated storage and handling systems and different identification codes. In addition, in the assembly phase, it is opportune to provide several reference and positioning systems for mounting the various aerofoils in the correct position, as designed.